Directional solidification of unstabilized HfO2--Ta

Aligned eutectic composites of unstabilized ZrO2 and Ta can be prepared crack-free even though the matrix has been completely transformed into the monoclinic lattice, i.e. the transformation-induced volume change does not result in the excessive cracking which is usually observed. Composites with Y2O3-stabilized ZrO2, however, crack extensively and exhibit inhomogeneous eutectic structures. Similar results are obtained with directionally solidified, stabilized ZrO2--W and stabilized HfO2--W. The fact that ZrO2 and HfO2 have modifications with identical crystal structures suggested that compositions of unstabilized HfO2 with Ta would also exhibit an improved solidification behavior compared with mixtures containing Y2O3-stabilized HfO2. Eutectic areas of HfO2--Ta sintered powder rods exhibited almost no cracking. There are two possible explanations for the fact that the composites with unstabilized ZrO2 and HfO2 survive the tetragonal→monoclinic transformation without cracking: (1) the matrix is prestressed by the Ta fibers when expanding at the lattice tranformation; or (2) since the composites are sintered in vacuum, ZrO2 or HfO2 are substoichiometric. This condition, together with Ta in solid solution, may cause the monoclinic lattice cell to exhibit a density similar to that of the tetragonal form. Both hypotheses, however, must be carefully investigated